Baghdad, April 12 (Reuters): A suicide bomber killed eight people in the Iraqi parliament today, slipping through multiple checkpoints in a brazen strike that challenged a major US-backed security crackdown in Baghdad.

US military spokesman Major-General William Caldwell said initial reports showed eight had been killed and 20 wounded in the blast which tore through a cafe where lawmakers were having lunch. Television said three of the dead were lawmakers.

It was the most serious breach of security in the Green Zone, the sprawling, heavily-protected area in central Baghdad that houses parliament, government offices and the US embassy.

President George W. Bush and Iraqi Prime Minister Nuri al-Maliki, who is on a trip to East Asia , condemned the attack, which Caldwell blamed on Sunni Islamist al Qaida.

The bold attack by a suicide bomber wearing an explosives vest came despite a two-month-old operation by thousands of US and Iraqi troops in the capital regarded as a last chance to stop a slide to full-scale sectarian civil war.

A truck bomb also killed at least seven people on Sarafiya bridge in northern Baghdad, a main artery linking east and west Baghdad, destroying most of the steel structure and sending several cars plunging into the Tigris river below.

How explosives were smuggled into the Green Zone is likely to be the focus of an investigation. They would have had to pass through an outer checkpoint manned by US and Iraqi troops and multiple inner checkpoints guarded by security contractors and foreign troops that are part of the US-led coalition.

“We are trying to backtrack all the systems to see how somebody was able to get a suicide vest into the Convention Centre where the members of parliament meet ... we are looking at who had access there,” Caldwell said.

The US military said this month that two explosives vests were found in the zone. A third suspected vest was known to have been missing and a hunt was launched to find it. Fouad al-Massoum, leader of the Kurdish bloc in parliament, said there was chaos after the explosion.

“Suddenly we heard a huge blast inside the restaurant. I saw a lot of MPs wounded and bleeding,” Massoum said.

Security officials, fearing there might be a second explosion, ordered everyone out of the building. But no one, including lawmakers, were allowed to leave the area straight after the blast so they could be questioned, officials said.

A Reuters witness said the explosion took place at the cashier’s register in the cafe, which is near parliament’s main assembly hall. Parliament was in session today.

“I saw a ball of fire and heard a huge, loud explosion. There were pieces of flesh floating in the air,” said the witness who was lightly wounded in the arm.

Officials named Mohammed Awadh, a member of the Accordance Front, the biggest Sunni bloc in parliament as one of the dead lawmakers.

Militants have rarely managed to penetrate the various checkpoints and carry out attacks inside the zone, although the area has come under increasing rocket and mortar attack in recent weeks.

A rocket landed close to a building where Maliki and UN secretary-general Ban Ki-moon were speaking last month, causing damage but no casualties.